A direct injection engine has been developed in order to improve fuel economy, reduce emission, and improve output. In such a direct injection engine, when an engine is at starting or at cold state (before warm-up), the fuel injected into the cylinder is hardly atomized and tends to adhere on an inner surface of a cylinder or a top surface of a piston. If such an adhering fuel is increased, a fuel quantity contributing to combustion is decreased so that the driving condition of the engine becomes unstable. The quantity of the adhering fuel is referred to as a wet quantity of the fuel hereinafter.
In a case where the engine is at starting or at cold state, if the fuel injection quantity is increased, the fuel contributing to the combustion is increased to obtain a stable combustion. However, if the fuel quantity is increased, the wet quantity of the fuel is also increased, which may increase HC emission especially when the warm-up of the engine has not been completed.
JP-2006-307736A (U.S. Pat. No. 7,222,602B2) shows a fuel injection system which reduces the wet quantity of the fuel. In this fuel injection system, when a deterioration of the fuel combustion is detected or estimated, the fuel injection is split into a plurality of injections to expedite the atomization of the fuel and reduce the wet quantity of the fuel.
When the engine is at starting or at cold state, an inner cylinder temperature is relatively low. Hence, even if a split injection is conducted, the injected fuel is not well atomized and the wet quantity is not reduced effectively, so that the emission is not sufficiently reduced.